This invention pertains to power transmission mechanisms of the type which include hydrokinetic couplings or torque converters and which further include so called lockup clutches which provide for a positive mechanical drive from the torque converter input shaft to the torque converter output shaft. Power transmissions which include torque converter lockup clutches are well known. Such clutch arrangements are usually employed to improve the efficiency of transmissions which include fluid couplings and particularly torque converter couplings which have the characteristic that as the ratio of the speed of the driven member (turbine) with respect to the driving member (impeller) approaches unity the efficiency of the fluid coupling decreases rapidly. The numerous arrangements of torque converter lockup clutches known in the art are generally characterized by a clutch actuating piston which is responsive to pressure fluid provided by a separate control circuit which further complicates the control circuitry of the transmission mechanism. Moreover, the structural arrangement of known lockup clutches for torque converters is generally somewhat complicated and tends to increase the overall size and weight of the transmission mechanism as well as the manufacturing cost.